Almatis-TU Delft Seminar on. Numerical Modeling of Rotary Kilns June 9, 2011 Room Vassiliadis, 16th floor, EWI Building, Mekelweg 4, Delft

Transcription

1 Almatis-TU Delft Seminar on Numerical Modeling of Rotary Kilns June 9, 2011 Room Vassiliadis, 16th floor, EWI Building, Mekelweg 4, Delft Organizers: M. Pisaroni (TU Delft), R. Sadi (Almatis) and D. Lahaye (TU Delft)

2 Calcium Aluminate Cements Calcium Aluminate Cements are very white, high purity hydraulic bonding agents providing controlled setting times and strength development for today's high performance refractory products. Almatis' calcium aluminate cements are for use in refractories designed for service up to 1800 C : Vibration and self-flowing castable mixes, Quick-setting mortar, joining mixtures, self-leveling floors, and compensation compounds Refractory castables for steel, nonferrous metals and petrochemicals industries.

3 Calcium Aluminate Cements Production The cement is made by fusing together a mixture of a calcium-bearing material (limestone) and an aluminium-bearing material. A typical kiln arrangement : Reverberatory furnace in which the hot exhaust gases pass upward as the lump raw material mix passes downward. The calcined material drops into the "cool end" of the kiln. The melt overflows the hot end of the furnace into a cooler in which it cools and solidifies. The cooled material are crushed and grounded. In the case of high-alumina refractory cements, where the mix only sinters, a rotary kiln must be used.

4 Rotary Kiln Philosophy Fundamentally, rotary kilns are heat excangers in which energy from hot gas phase is extracted by the bed material. During the passage, various heat exchange processes: drying, heating, and chemical reactions that cover a broad range of temperatures. Cylindrical vessel, inclined slightly to the horizontal, which is rotated slowly about its axis. The material is fed into the upper end of the cylinder. As the kiln rotates, material gradually moves down towards the lower end, and may undergo a certain amount of stirring and mixing. Hot gases: opposite direction as the process material. The hot gases is be generated by a flame projected from a burner-pipe inside the kiln.

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9 MODELING APPROACH STEP1: Heat transfer in the lining (2D) Combustion Model (1D) Setup and Test STEP2: Combustion and heat transfer in the gas and in the lining (3D) Nox Temperature STEP3: Granular material thermal and motion analysis Chemical

10 TURBULENT NON-PREMIXED COMBUSTION IN A ROTARY KILN AIR INLET CONVECTION/CONDUCTION/RADIATION NON PREMIXED COMBUSTION POLLUTANT EMISSION TURBULENCE BURNER MIXING AND TRANSPORT OF CHEMICAL SPECIES

11 TURBULENT NON-PREMIXED COMBUSTION IN A ROTARY KILN CONVECTION/CONDUCTION/RADIATION NON PREMIXED COMBUSTION POLLUTANT EMISSION TURBULENCE BURNER MIXING AND TRANSPORT OF CHEMICAL SPECIES

12 TURBULENT NON-PREMIXED COMBUSTION IN A ROTARY KILN CONVECTION/CONDUCTION/RADIATION NON PREMIXED COMBUSTION POLLUTANT EMISSION TURBULENCE MIXING AND TRANSPORT OF CHEMICAL SPECIES ROTATING WALL (REFRACTORY)

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15 THE COMBUSTION Combustion Air Natural Gas Cooling Air A/G ratio : [m3/h] 425 [m3/h] 300 [m3/h] TEMP C C C COMPOSITION 23 % O % CH4 23 % O2 5.6 % C2H % C3H8 0.9 % C4H % CO % N2

16 COMBUSTION MECHANISMS: FAST : 6 SPECIES, 1 REACTION H2 CO2 H2O O2 N2 CH4 CH4 +2O2 CO2 + 2H2O DETAILED MECHANISM WITH INTERMEDIATE REACTIONS: MODEL 1: 12 STEP 16 SPECIES MODEL 2: 15 STEP 19 SPECIES DETAILED MECHANISM WITH MORE SPECIES MODEL 3: 463 REACTIONS 70 SPECIES MODEL 4: N-ALKANES (VERY DETAILED) ARRHENIUS COEFFICIENTS: dependence of the rate constant k of Chemical reactions on the temperature T and activation energy E a.

17 COMBUSTION MODEL: Eddy Break-Up (EBU) Models Track individual mean species concentrations on the grid through transport equations. The reaction rates are calculated as functions of : o mean species concentrations, o turbulence characteristics o temperature. A mean enthalpy equation is solved. The mean temperature, density and viscosity are then calculated Mean enthalpy Species concentrations Standard EBU Model : reaction rate is dictated by the turbulent mixing time scale. Individual species are transported at different rates according to their own governing equations. The reaction rate model: expression that takes the turbulent micromixing process into account. Hybrid Kinetics/EBU Model assumes that the reaction rate is also affected by finite-rate chemical kinetics.

18 TURBULENCE MODELS: K-Epsilon turbulence model: transport equations are solved for the turbulent kinetic energy and its dissipation rate. Realizable Two-Layer K-Epsilon model combines Realizable K-Epsilon model: different transport equation for epsilon. Critical coefficient of the model, is expressed as a function of mean flow and turbulence properties. Two-layer approach: alternative to the low-reynolds number approach. The computation is divided into two layers. (Shear driven Wolfstein model) Layer adjacent to the wall: turbulent dissipation rate and turbulent viscosity are functions of wall distance. Entire flow: equation for the turbulent kinetic energy.

19 FLOW MODELS: The Ideal Gas (with combustion) model allows thermodynamic polynomial for calculation of specific heat of individual components of the mixture. Multi-Component Gas model used to simulate a miscible mixture of two or more pure gases (MECHANISM). Segregated Flow model solves the flow equations (one for each component of velocity, and one for pressure) in a segregated (uncoupled) manner. Linkage between momentum and continuity equations is achieved with a predictorcorrector approach (Rhie-and-Chow-type pressure-velocity coupling combined with SIMPLE algorithm). Segregated Fluid Enthalpy model solves the total energy equation with chemical thermal enthalpy as the independent variable. Temperature is then computed from enthalpy according to the equation of state. Segregated Species model solves the species continuity equations for a multi-component fluid mixture. Together with global mass continuity, these equations provide a means for updating the field of N mass fractions defining the mixture composition.

20 OTHER MODELS: Participating Media Radiation Model Nox Emission model provides a framework for modeling the NOx transport equation. Nox Zeldovich Model NOx is considered to be a passive scalar, not influencing density calculations. Therefore, in steady-state simulations, the Nox Emission model should be used as a post-processing tool (it should be activated after the flow field has converged). SOLVERS

21 MESH

22 MESH

23 TEMPERATURE PROFILE

24 TEMPERATURE PROFILE

25 TEMPERATURE PROFILE

26 TEMPERATURE PROFILE

27 CH4 PROFILE

28 O2 PROFILE

29 CO2 PROFILE

30 GAS-SOLID INTERFACE TEMPERATURE

31 INCIDENT RADIATION

32 INCIDENT RADIATION

33 INCIDENT RADIATION

34 RADIATIVE ABSORPTION

35 RADIATIVE ABSORPTION

36 RADIATIVE ABSORPTION

37 TURBULENT LENGTH SCALE

38 VELOCITY VECTORS

39 VELOCITY VECTORS

40 END OF PART 1 The power of Fire, or Flame, for instance, which we designate by some trivial chemical name, thereby hiding from ourselves the essential character of wonder that dwells in it as in all things, is with these old Northmen, Loke, a most swift subtle Demon of the brood of the Jötuns... From us too no Chemistry, if it had not Stupidity to help it, would hide that Flame is a wonder. What is Flame? Carlyle on Heroes Odin and Scandinavian Mythology

41 Realizable K-Epsilon model Standard K-Epsilon model BACK

42 Standard EBU Individual species transported at different rates according to their own governing equations derived from the instantaneous governing equations for species i: Time average of the instantaneous governing equation: Reaction rate is modeled is modeled through an expression that takes the turbulent micromixing process into account. BACK

43 Hybrid EBU This model accounts for finite rate effects by assuming that the actual reaction rate is the minimum of the reaction rates from (STANDARD) and Hence BACK

44 SOLVERS: Wall distance SIMPLE [Semi-Implicit Method for Pressure-Linked Equations] Segregated species Segregated energy K-Epsilon Turbulence Solver K-Epsilon Turbulence Viscosity Solver Nox solver BACK